Cellular mobile station system

ABSTRACT

A cellular mobile station system wherein a handover decision is based on measuring downlink transmissions of a serving cell and neighboring cells by a mobile station. Adjacent cells are typically arranged to be partly overlapping so as to ensure there is a sufficient period of time for the reception and decoding of the downlink transmissions of the neighboring cells, as well as for a handover decision. In some of the cells, there may be terrain obstacles, buildings, tunnels or the like obstacles shadowing the downlink transmission of the neighboring cell so that overlapping is not sufficient between such a cell and a neighboring cell. A repeater or an antenna is provided for extending the coverage area of the downlink transmission of the shadowed neighboring cell in such a cell so as to allow an advanced transmission and decoding of the downlink transmission of the shadowed neighboring cell.

This application is the national phase of international applicationPCT/F195/00676, filed Dec. 12, 1995 which was designated the U.S.

1. Field of the Invention

The invention relates to a cellular mobile station system wherein ahandover decision is based on measuring downlink transmissions of aserving cell and neighboring cells by a mobile station. Adjacent cellsare typically arranged to be in a partially overlapping configuration soas to ensure that there is a sufficient period of time for the receptionand decoding of the downlink transmissions of the neighboring cells fora handover decision.

2. Background of the invention

In cellular mobile station systems, a mobile station, in which a call isestablished, that is, which is using a traffic channel, measuresconstantly the quality and/or the strength of the downlink signal from aserving base station. The serving base station also informs the mobilestation of the neighboring cells/base stations and of their controlchannel frequencies. On the basis of this information, the mobilestation is able to measure also the control channel frequencies of theneighboring base stations during a call. The measuring results arestored into the mobile station and/or are transmitted to the mobilestation network for the handover decision depending on whether thehandover decision is made by the mobile station or by the mobile stationnetwork. A handover refers to a switching of a call from one trafficchannel to another during a call.

The base stations of the cellular network constantly broadcastinformation about themselves and their surroundings. This informationincludes a cell or base station identifier, based on which the mobilestation is able to identify the neighboring cell to be measured. Thecell or base station identifier is also transmitted to the cellularnetwork in possible measuring reports. The mobile station has to decodethe cell or base station identifier of a neighboring cell before thiscell can be used as a potential target cell for a handover. In orderthat the mobile station would have a sufficient period of time to decodethe cell or base station identifier and that way allow the handover, theadjacent cells of the cellular mobile station system must haveoverlapping areas. Besides the acceptable decoding time of the cell orbase station identifier, the size required of the overlapping area oftwo adjacent cells depends, e.g., on the speed of the mobile station andthe parameters of handover algorithm, such as, the size of the timewindow used for averaging the measured signal levels of the neighboringcell, for example.

If the overlapping area of two adjacent cells is so small that thehandover cannot be made, the call is lost. One reason for a lost call isan undecoded cell or base station identifier of the neighboring cell.The overlapping requirement of cells is significant especially inmicrocells in which the diameter is some hundreds of meters. It is,however, difficult to arrange a sufficiently large overlapping area inplaces where terrain obstacles, buildings or the like obstaclesobstructing the propagation of radio signals are shadowing thetransmission of the neighboring cell. Such problematic places are found,for example, in tunnels, at street corners and inside buildings. Forexample, when a mobile station comes fast out of a tunnel, theneighboring cell outside the tunnel may develop an immediate need for ahandover but the cell or base station identifier of the neighboring celloutside the tunnel is undecoded and the call is lost because a handoverto the neighboring cell cannot be made. This situation is illustrated inmore detail by means of an example with reference to FIG. 1.

In FIG. 1 the serving cell is indicated by the reference character S andthe neighboring cell by the reference character A. The serving cell is,for example obstructed by a tunnel 3 so that cell S extends somewhatoutside the tunnel at the opening of the tunnel. The neighboring cell Ais a cell outside the tunnel and the radio coverage of cell A does notextend essentially inside the tunnel. Thus, cells S and A have a verysmall overlapping area OA. A mobile station MS measures all theneighboring cells specified for the serving cell S and transmits themeasuring results to the cellular network. The measuring resultincludes, e.g., the field strength of the downlink signal of theneighboring cell and the cell or base station identifier of theneighboring cell. This information about the neighboring cell istransmitted to the cellular network if the mobile station is able toreceive the downlink signal of said cell. When the MS reaches the radiocoverage of cell A in position (1), it can receive the downlink signalof cell A and start decoding the cell or base station identifier of cellA. As it has not yet been possible for the mobile station MS to decodethe cell or base station identifier, the information about theneighboring cell is insignificant in the measuring result as it cannotbe made certain that the measurings concern the correct cell, for whichreason the neighboring cell cannot be used as a target cell in a case ofa handover. In position (2), the cellular network detects the need for ahandover, for example, due to the low reception level of the servingcell S. If the overlapping area OA of cells S and A is so small that themobile station MS reaches the coverage area of cell S at position (2)before the base station identifier of the neighboring cell A is decoded,a handover to cell A cannot be made and the call will be lost.

At present in cellular systems, there are some solutions for eliminatingthis problem. The size of the cell can be enlarged and antennas can bedirected so that required cell overlapping is achieved. These solutionscause some other problems, especially in network planning. Theenlargement in the coverage area of cell S can cause co-channelinterference in some parts of the network. In a case in which thecoverage area of cell A is enlarged, the advantage gained by microcellsis lost, besides which co-channel interference may occur. The openingsproduced by directing the antenna of cell A towards cell S have to befilled from other cells.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a new method forrelieving or eliminating the problem mentioned above.

This is achieved with a cellular mobile station system disclosed in theforegoing Background section which is according to the inventioncharacterized in that a first cell, in which terrain obstacles,buildings, tunnels or the like obstacles obstructing the propagation ofradio signals are shadowing the downlink transmission of one of theneighboring cells so that overlapping is not sufficient between thefirst cell and a respective one of the neighboring cells, is providedwith a repeater or an antenna of the respective one of the neighboringcells which extends in the first cell the coverage area of the downlinktransmission of the respective one of the neighboring cells for anadvanced transmission and decoding of the downlink transmission of therespective one of the neighboring cells.

The basic idea of the present invention is to render it possible for amobile station to detect a neighboring cell and decode a cellidentifier, such as a base station identifier before the mobile stationcan listen to the actual downlink transmission of the neighboring cell.Because of this, the size requirements of the overlapping areas of cellsare not as great in relation to the actual coverage areas as especiallyin the microcell areas of the prior art solutions. The advanceddetection and decoding of a neighboring cell according to the inventionis brought about by extending the coverage area of the neighboring cellto the area of the cell serving only in the downlink direction in whichthe obstacles obstructing the propagation of radio signals are shadowingthe downlink transmission of the neighboring cell. As the object of theextension of the neighboring cell is only to prepare for a fasthandover, an equivalent extension need not be made in the uplinkdirection. The advanced detection of a neighboring cell and the decodingof the base station identifier according to the invention allow theneighboring cell to be selected as a potential target cell of ahandover, even before arriving in the actual coverage area of theneighboring cell, and thereby ensuring a fast handover and maintainingthe call, even if there would not be enough time to decode the basestation identifier from the actual downlink transmission of theneighboring cell. The invention can be implemented over existing networkplanning without having to change the cells of the existing networkplanning in any way. This is a significant advantage, because,especially in a microcell environment, it is difficult to redo networkplanning, for example, the extension of cells may disturb othermicrocells.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail by means of preferredembodiments with reference to the accompanying drawing, in which:

FIG. 1 illustrates a handover situation in a mobile station system ofthe prior art when moving from a cell inside the tunnel to a celloutside the tunnel, and

FIG. 2 illustrates a cellular mobile station system in which theextension of the downlink direction of the neighboring cell according tothe invention is accomplished in the tunnel cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is applicable to any cellular mobile stationnetwork in which the mobile station performs the measuring ofneighboring cells for handover and requires sufficient overlapping ofthe coverage area of the cells so as to ensure there is a sufficientperiod of time for the reception and decoding of the downlink signal ofthe neighboring cell. In the following, the invention will be describedby using the GSM mobile station system as an example without restrictingthe invention thereto.

GSM (Global System for Mobile-Communications) is a pan-European digitalmobile station system. FIG. 2 discloses only some basic structures ofthe GSM system, without further specifying the properties and otherparts of the system. With regard to a more detailed description of theGSM system, a reference is made to the GSM Specifications and to The GSMSystem for Mobile Communications, by M. Mouly & M. Pautet, Palaiseau,France, 1992, ISBN:2-9507190-0-0-7.

A mobile services switching center MSC attends to the switching ofincoming and outgoing calls. It performs similar tasks to a publicswitched telephone network center. In addition to the above, it performsfunctions characteristic of only mobile telephone traffic, such assubscriber location management. Mobile radio stations or mobile stationsMS are connected to the center MSC by means of base station systems BSS.The base station system BSS comprises a base station controller BSC andbase stations BTS. One base station controller BSC is used forcontrolling several base stations BTS. A function of the BSC is, e.g.,handovers in cases in which a handover is made inside the base stationor between two base stations that are both under the control of the sameBSC. For the sake of clarity, only one base station system BSS is shownin FIG. 2, in which two base stations BTS1 and BTS2 are connected to thebase station controller BSC. The radio coverage areas of the basestations comprise corresponding radio cells S and A.

The mobile station MS with an ongoing call constantly measures thequality or level of the signal received from the serving cell and thereception level of the control channel frequencies of neighboring cellsin order to prepare for handovers. The serving cell notifies the mobilestation which neighboring cells it should measure. In the GSM system,the control channel carrier waves the mobile stations listen to andmeasure are called broadcast control channels BCCH. In order that themobile station MS can distinguish the cell with the same BCCH carrierwave, a base station identity code BSIC is included in the BCCHtransmission. It is possible that the MS receives in some cases two BCCHtransmissions from different cells on the same carrier wave frequency.As was mentioned earlier, the mobile station MS is provided with a listof the carrier wave frequencies it should observe for measuringneighboring cells. In a report message, which the mobile stationregularly transmits to the serving base station BTS and which containsat most the measuring results of six neighboring cells, the mobilestation has to inform the BSIC of every measured BCCH carrier wave onwhich it reports measuring results. This means that the mobile stationhas to decode a synchronization channel SCH on the BCCH carrier wave tofind out the BSIC of the neighboring cell. The base station controllerBSC, to which the base station BTS transmits the measuring reports fromthe mobile station MS, is able to check in uncertain cases which cellhas actually been measured. If the BSIC of the BCCH carrier wave has notbeen decoded, the measuring results of said BCCH carrier wave will notbe reported further and the neighboring cell related thereto cannot beselected as a potential target cell of handover.

In the cellular system of FIG. 2, cell S of the base station BTS1 is acell obstructed inside a tunnel 23 the coverage area of which extendsonly a little outside the opening of the tunnel. Correspondingly, cell Aof the base station BTS2 is a cell outside the tunnel 23 the coveragearea of which does not at all extend inside the tunnel 23. Thus theoverlapping area OA of cells S and A is very small and it does notprovide the mobile station MS with enough time in all situations tomeasure and decode the BCCH carrier wave of the base station BTS2 whenthe mobile station MS is moving from the tunnel to the overlapping areaOA. This may be followed by a handover failure and a loss of the call,as was explained in connection with FIG. 1.

In accordance with the invention, an extended coverage area A' is formedfor the downlink BCCH signal inside cell S in the area in which thetunnel 23 obstructs the extent of the actual coverage area A of the basestation BTS2. This extension area A' is achieved with a repeater and adirectional antenna 22 which are positioned inside the tunnel 23 andcell S outside cell A. The repeater and antenna 22 are connected to thebase station BTS2 by means of cabling 21. The downlink BCCH signal ofcell A is transmitted in the extension area A' at such a level that themobile station in this area can decode the identifier BSIC of cell A.Further, the repeater and antenna 22 are positioned so that cell A andthe extension A' do not significantly cause any disturbance to oneanother. The position of the repeater and antenna 22 in the extensionarea A' can be found by measuring the signal level of cell A, forexample.

In the following, the effect of the extension area A' on handover isexamined by means of an example. It is first assumed that the mobilestation MS is connected to the serving cell S. When the mobile stationMS moves towards the neighboring cell A, it will reach the extensionarea A' of the neighboring cell at position (3). At this position themobile station MS is able to receive the BCCH carrier wave of cell Awhich is transmitted through the repeater and antenna 22 and thus isready to start decoding the BSIC. When the BSIC is decoded, the MS willstore the BSIC. When this BCCH carrier wave is no longer among the sixstrongest carrier waves, the BSIC should be retained for at least 10seconds in compliance with the GSM Specification 0508. This time isequivalent to a distance of about 140 meters at a speed of 50 km/h. Whenthe mobile station MS reaches the actual coverage area of theneighboring cell A at position (1), the identifier BSIC of theneighboring cell A is already decoded because of the extension area A'.Therefore in the case of a handover, cell A can be regarded as apossible target cell. If the mobile station MS moves fast in the area ofcells S and A, it is possible to maintain the BSIC stored in the memoryas long as is needed for moving from the area A' to the area A. If themobile station MS moves slowly, it can lose the BSIC information betweenthe areas A' and A but decode it again in the overlapping area OA ofcells S and A. The mobile station transmits measuring reports via the isbase station BTS1 to the base station controller BSC.

The base station controller BSC detects a need for a handover atposition (2). This need can be detected for example on the basis of thelow reception level of the serving cell S. When the need for a handoveris detected, the call is transferred to a suitable neighboring cell. Inthis case, the target cell is cell A which has been found to be asuitable neighboring cell for a handover. This means that cell A hasbeen identified by means of BSIC decoding and the reception level ofcell A is sufficient for maintaining the call.

The invention is above described by using as an example cell Spositioned in the tunnel. The invention is not in any way restricted tothis example, but it can be applied in any shadow area of the network toeliminate the same problem. Cell S can for example be a cell inside abuilding and cell A a cell outside the building at the entrance. In thatcase, the extension area A' can be situated inside the building in thelobby before the entrance, for example. Further, cell S can be a cellinside a multi-story car-parking structure and cell A a cell inside theparking structure at the entrance. In that case, the extension area A'can be situated inside the parking structure before the exit, forexample. Further, cell S can be a cell between tall buildings coveringthe street between them and cell A a cell covering an intersectingstreet.

When needed, the extension area can be formed by using one antenna,several antennas, a combination of an antenna and a repeater, a leaking(radiating) cable or any equipment with which a suitable type ofdownlink extension area is achieved in each special case.

The figures and the explanation related is thereto are only meant toillustrate the present invention. It is to be understood that changescan be made to the disclosed cellular system without deviating from thespirit and scope of the accompanying claims.

I claim:
 1. A cellular mobile communications system in which a handoverdecision of a mobile station from a serving cell having a coverage areato a neighboring cell from among a plurality of other cells, and saidneighboring cell having a basic coverage area which at least partiallyoverlaps said said coverage area of said cell, and existence of at leastone obstacle shadows downlink transmission capability of saidneighboring cell so as otherwise to effectively cause insufficientoverlapping of said serving and said neighboring cell to ensure thatthere is a sufficient period of time for reception and decoding ofdownlink transmissions of said neighboring cell and for executing adecision to handover said mobile station from said serving cell to saidneighboring cell, said system further comprising:at least one of arepeater and an antenna so arranged as to effectively extend said basiccoverage area for said downlink transmissions of said neighboring cellby an extended coverage area into said coverage area of said servingcell, for advancing transmission and decoding downlink transmissions ofsaid neighboring cell.
 2. The cellular mobile communications system ofclaim 1, wherein;said at least one of a repeater and an antenna is soarranged as to advance recognition of said neighboring cell as apotential target for a handover of said mobile station from said servingcell.
 3. The cellular mobile communication system of claim 1,wherein:said extended coverage area of said neighboring cell isnon-overlapping in relation to said basic coverage area of saidneighboring cell.
 4. The cellular mobile communications system of claim1, wherein:said coverage area of said first cell extends from a basestation primarily serving within a building, and said basic coveragearea of said neighboring cell extends from a basic station primarilyserving outside said building.
 5. The cellular mobile communicationssystem of claim 1, wherein:said coverage area of said first cell extendsfrom a base station primarily serving within a tunnel, and said coveragearea of said neighboring cell extends from a base station primarilyserving outside said tunnel.
 6. The cellular mobile communicationssystem of claim 1, wherein:said at least one of a repeater and anantenna comprises a leaking cable.
 7. The cellular mobile communicationssystem of claim 1, wherein:said at least one obstacle is constituted byat least one of at least one building, at least one tunnel and at leastone terrain feature.